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Luminescence dating methods determine the time elapsed since the last exposure of minerals to sunlight (or heat).

Optically stimulated luminescence. Optically stimulated luminescence (OSL) (57) measures the time of deposition of sediments. Luminescence signals are linked with natural ionizing radiation because natural crystals behave as natural dosimeters: They record the irradiation doses to which they are exposed and can deliver, when stimulated, a signal correlated to the total dose they absorbed. The method requires the determination of two quantities: the equivalent dose (De), on one hand, corresponding to the total irradiation dose absorbed by minerals since their last zeroing (when bleached by sunlight at the time of deposition), obtained by luminescence measurements. The dose rate (Dr), on the other hand, corresponds to the dose absorbed per unit time, which is largely the product of radioactivity within an area 30 to 50 cm around the sample. It is determined by measurements of radioelements concentration in the laboratory, combined with in situ dosimetric measurements.

Feldspars IRSL. Feldspars IRSL dating requires, contrary to quartz OSL dating, considering anomalous fading (a loss of charge from stable traps) or using protocols to overcome it. Laboratory-measured fading rates can be used to correct ages (58). The post-infrared IRSL (pIRIR) signal, measured at elevated temperature (e.g., 290°C), can also be used to avoid anomalous fading effects and lead to accurate ages (59, 60).

Sampling and analyses. Six sediment samples were collected from the DG-A/001 stratigraphic sequence in 2016–2017 and dated in the Bordeaux Montaigne University Luminescence Laboratory of the Centre de Recherche en Physique Appliquée à l’Archéologie (CRP2A), a laboratory with long experience of dating Paleolithic sites (6163). All samples were collected at night, under controlled red lighting, by excavating sediment from the trench section. Subsamples were collected in all cases for radioelement contents measurements. Dosimeters (aluminum tubes), containing three Al2O3:C crystal chips were inserted into the stratigraphic profiles at the exact location of the luminescence samples to measure gamma and cosmic dose rates. These dosimeters remained buried for a year, after which they were also measured at the CRP2A (64).

Each sample was prepared mechanically and chemically in the conventional manner (65). The first tests with the quartz fraction indicated that the quartz was not suitable for luminescence measurement: No OSL (neither natural nor regenerated) signal could be measured. Conversely, the K-feldspar fraction was dated using an adapted SAR (Single-Aliquot Regenerative Dose) protocol (66, 67) using two different signals: (i) the IR50 signal, corresponding to the signal measured during a stimulation at 50°C, which is affected by anomalous fading (68). To correct the results from this phenomenon, g values were measured for all aliquots, and the DRC (Dose Rate Correction) (68) was applied; (ii) the pIRIR290 signal was measured during a stimulation at high temperature (290°C) after a first stimulation at 50°C (69, 70).

During exposition to sunlight in nature, the IR50 signal is bleached faster than the pIRIR290 signal because the latter signal from more distant electron-hole pairs (71). However, the pIRIR290 signal does not seem to be affected by anomalous fading (69, 70, 72).

In the present work, all six samples were dated with pIRIR290 signal measurements, based on 10 to 12 aliquots for each sample; for younger sediment samples in the present study, IR50 age estimates (based only on three aliquots for each sample) were obtained after fading correction. For older samples (when approaching the field saturation level of the IR50 signal), the fading correction is no longer possible.

pIRIR290 ages have been determined using the ADM (Average Dose Model) (73) and are presented in table S1; they are in good agreement, within uncertainties, with the stratigraphy (Fig. 2). IR50 ages are presented in table S2 and are consistent with IRpIR290 ages within uncertainties (2σ).

Note that these experiments allow dating of the last exposure of the feldspar grains to light; in sites with complicated taphonomic histories, similar to the present one, only terminus post quem and TAQ can initially be deduced from luminescence dating results (36). In this specific case, the fact that no high dispersion of De values has been detected for any of the samples [De SDs vary between 3 and 7%; see table S1 for the overdispersion values calculated with the Central Age model (74)]. Even when measuring small aliquots (1 mm in diameter), this allows us to hypothesize a unique deposition event for all grains (same last time of light exposure). The De distributions are presented in the radial plots in fig. S7 and show very low dispersion in the data. This dated moment can be contemporaneous with human occupation or with reworking of one or several sedimentary levels containing one or several archaeological assemblages (during which either light exposure led to a complete signal resetting or to no resetting at all). Moreover, sample SNAP16-1 came from an aeolian deposited sand layer, suggesting that exposure to sunlight most likely was sufficient to fully reset the pIRIR290 signal. The observation that the three colluvial levels (SNAP16-1 to SNAP16-3) above it in the stratigraphy simultaneously displayed similar dispersion in De values and appeared younger in age than the well-bleached aeolian level reinforces the hypothesis that bleaching of the pIRIR290 signals was complete during the deposition of the colluvial layers at the site. IR50 age estimates (even if based on few aliquots) and their congruence with pIRIR290 ages also confirmed that no partial bleaching needs to be considered.

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